Arteries and veins provide the pathways through which blood is circulated to all parts of the human body. The heart, a well-developed muscle, pumps blood through these established pathways, the blood providing needed oxygen and removing unwanted carbon dioxide. Blood containing carbon dioxide is carried to the heart by the veins where it is pumped from the right atrium through the tricuspid valve to the right ventricle. Blood is then pumped through the pulmonary valve to the lungs by the arteries where carbon dioxide is exchanged for oxygen. Oxygenated blood returns to the left atrium and is pumped through the mitral valve to the left ventricle. From the left ventricle the blood is pumped through the aortic valve and then throughout the body. Supplying oxygen and removing carbon dioxide, the blood returns by the veins to the right atrium. This process is continually repeated as the heart meets the body's need for a continuous supply of oxygen and concomitant need for removal of waste products.
In response to the transvalvular pressure gradient between systole and diastole, the cardiac valves open or close to either permit blood flow or prevent backflow. Congenital abnormalities or valve disease, which may be caused for example by calcific degeneration, rheumatic degeneration or endocarditis, may interfere with valvular performance. Incompetence, or failure of the valve to completely close, results in regurgitation with a correspondingly increased effort by the heart to pump sufficient amounts of blood to the body and lungs. Stenosis, which is a narrowing of the valve orifice, also stresses the heart as it attempts to pump an adequate volume of blood through a smaller opening. In cases involving mitral or aortic stenosis, surgical treatment may involve reconstruction of the valve by mitral commissurotonomy or valvuloplasty. Complications after surgery include inadequate relief of stenosis due to severe subvalvular stenosis or mitral regurgitation due to excessive commissurotonomy. These procedures, with potential associated complications, may be appropriate in a limited number of cases, however, reconstruction of an aortic valve or stenotic mitral heart valve normally is not feasible, and the surgeon must replace the defective valve with a prosthetic one. Reconstruction of an incompetent mitral valve is a common procedure, but in some cases the mitral valve pathology is so severe replacement is required. Of the four heart valves, the most commonly replaced are the mitral valve and the aortic valve. For information on aortic valve surgery and mitral valve surgery, see:
1.) Wisman, Craig B. and John A. Waldhausen. "Aortic Valve Surgery." Complications in Cardiothoracic Surgery. Ed. John A. Wiseman and Mark B. Orringer. St. Louis: Mosby Year Book, date. 237-247. PA1 2.) Cohn, Lawrence H. "Management of Complications Related to Mitral Valve Surgery." Complications in Cardiothoracic Surgery. Ed. John A. Wiseman and Mark B. Orringer. St. Louis: Mosby Year Book, date. 248-257. PA1 3.) Fann, James I., Carlos E. Moreno-Cabral, and D. Craig Miller. "Caged-Ball Valves: The Starr-Edwards and Smeloff-Sufter Prostheses." Replacement Cardiac Valves. Ed. Endre Bodnar and Robert Frater. New York: Pergamon Press, 1991. 149-186. PA1 4.) Lefrak, Edward A. and Albert Starr, eds. Cardiac Valve Prostheses. New York: Appleton-Century-Crofts, date. 3-32. PA1 5.) McGoon, D. C. "The Status of Prosthetic Cardiac Valves." Biological Tissue in Heart Valve Replacement. Ed. M. I. Ionescu, D. N. Ross, and G. H. Wooler. London: Butterworths, date. 3-17. PA1 6.) Bain, William H., and S. A. M. Nashef. "Tilting Disk Valves." Replacement Cardiac Valves. Ed. Endre Bodnar and Robert Frater. New York: Pergamon Press, 1991. 187-200. PA1 7.) Horstkotte, Dieter and Endre Bodnar. "Bileaflet Valves." Replacement Cardiac Valves. Ed. Endre Bodnar and Robert Frater. New York: Pergamon Press, 1991. 201-228. PA1 8.) Krieger, Karl H. and O. Wayne Isom, eds. Blood Conservation in Cardiac Surgery. City: Springer, 1997. PA1 9.) U.S. Pat. No. 4,506,394, by Pierre Bedard, entitled "Cardiac Valve Prosthesis Holder," issued Mar. 26, 1985.
Two types of prosthetic valves are currently available in cases where surgical treatment requires replacement of an aortic or mitral valve. These are bioprosthetics and mechanical prosthetics. Bioprosthetics are generally made from porcine or bovine material that may be attached to a metal or plastic frame. Although bioprosthetics are less thrombogenic than mechanical prostheses, they degenerate over time and must be replaced with another prosthetic device. Because of uncertain life expectancy and the associated need for replacement of bioprosthetics, mechanical prosthetics generally are preferred. Many mechanical prosthetic devices have been utilized since the 1960's with varying degrees of success. The development of these mechanical valves can be traced by reviewing the general classes or categories of mechanical prostheses.
Working independently, Hufnagel and Campbell designed the first prosthetic valve, which consisted of a Lucite tube and a mobile spherical poppet. Hufnagel was the first to use such a valve in the descending thoracic aorta of a human. Following the success of Hufnagel, the next advance in mechanical valves was the introduction of the caged-ball valves. Starr-Edwards first inserted such a valve in the mitral position, while Harken first positioned the valve in the aortic position. Caged-ball valves consist of generally three or four struts attached to a ring that is inserted in either the mitral or the aortic position. Contained within the valve struts is a silastic ball, which controls the flow of blood through the valve. To prevent thrombosis, which begins on the atrial surface of the device, a retractable Silastic shield was added to cover the suture line in the dog model, but this Silastic shield was abandoned when this valve was implanted in humans. A number of caged ball valves followed in the wake of the Starr-Edwards and Harken devices including the Serville-Arbonville, Harken-Daval, Cooley-Cromie, Debakey-Surgitool, and Smeloff-Cutter. These models were succeeded by improved designs and have been taken off the market. The Starr-Edwards caged-ball valve is still used today.
Caged-ball valves belong in the class of lateral flow valves. Also developed at about the same time is a class of valves known as central flow valves, which are designed to imitate natural valves. Unable to endure the stress of repeated flexion, the materials available to create these valves did not prove sufficiently durable. Until more durable materials are developed, central flow valves have, by in large, been abandoned in favor of other types of lateral flow valves such as the tilting disk valves and the bileaflet valves.
Compared to the caged-ball valves, tilting disk valves are designed to reduce the bulk of the earlier valves while improving hemodynamic characteristics. Tilting disk valves have a lower profile than caged-ball valves. Instead of a ball, these valves utilize a disk which is tilted such that blood flow is less obstructed in an open position. Most commonly used until 1982 was the Bjork-Shiley valve. This valve has a free-floating disk retained by a low-profile M-shaped strut on the inflow side and a U-shaped strut on the outflow side of the valve. The original version is capable of pivoting to an opening of 60 degrees, while later versions provided a convexoconcave shape and were available with an opening of either 60 or 70 degrees. However, a high incidence of failure due to breakage of the metal struts led to discontinued use of this valve. Today, the Omniscience and the Medtronic Hall valves are the most commonly used tilting disk valves in the United States.
Another type of low-profile heart valve prostheses is the bileaflet valve. First designed by Gott-Daggett, this valve replaces the ball or disk of the prior valves with two semicircular leaflets retained within the ring by two struts. While providing improved hemodynamics, the Goft-Daggeft lacked proper wash-out which made the device highly thrombogenic. The Gott-Gaggett valve is best known for being the impetus for the development of pyrolitic carbon, in particular, Pyrolite.RTM.. Pyrolite is a thromobresistant material that is particularly biocompatible. At the present time, Pyrolite, or other pyrolitic carbon material, is used to construct bileaflet valves, as well as the disks and one orifice design of the tilted disk valves. The most commonly implanted bileaflet valves are the Sultz-Carbomedics and St. Jude valves. For information concerning bileaflet valves, see
Regardless of the type of prosthesis chosen, a similar procedure is followed for all mechanical valve implantations. First, the surgeon exposes the heart. Then, the aorta is clamped to cut-off the blood flow to the heart, and a heart-lung machine performs the heart's normal function, supplying blood to the rest of the body. After connecting the heart to the machine, the defective or diseased valve is exposed by making an incision in the aorta and the left atrium. Excision of the diseased tissue leaflets of the valve is then performed along with debridement of calcium using a Kelly clamp or pituitary rongeurs. After removing the natural valve leaflets, the surgeon places sutures through the surrounding heart valve tissue around the heart valve, drawing the sutures out of the chest of the patient. Everting sutures sometimes are used to prevent interference of the mechanical leaflets by retained native valve structures, chordal structures or by tips of papillary muscles. These sutures push the annulus of the prosthetic device away from the underlying tissue. Each suture is then drawn by needle through the cloth sewing ring that surrounds the replacement valve. To properly position the replacement valve, it is pressed down along the sutures, which act as guide wires. In addition to the use of everting sutures, proper positioning avoids interference with the device's moving parts. This is particularly true for valves such as the original St. Jude valve, which cannot be repositioned once sutured into place. When in the correct position, each suture is tied off in a knot with the end being trimmed.
There are a number of risks associated with open-heart surgery that are related to the duration of the procedure. Despite advances in blood conservation, the longer it takes to complete a surgical procedure the greater the chance the patient may need a transfusion, which carries with it the risk of transmitting diseases such as HIV or hepatitis, among others. Further, as noted in one text on blood conservation in cardiac surgery, " . . . a perfect operation in a patient that bleeds to death is a catastrophe." In addition to problems associated with blood loss and transfusion, a patient connected to a heart-lung machine during a valve replacement procedure may suffer memory loss after surgery. See
Other risks associated with mechanical valve procedures are paravalvular leakage and the need for subsequent replacements. Such leaks may be avoided by proper preparation of the heart valve tissue before placement of the prosthetic device, but if not successfully avoided paravalvular leaks result in excessive cardiac workload and the possibility of haemolysis with resulting anaemia.
A prosthetic valve suffering from excessive wear or mechanical failure must be replaced in a subsequent surgery. In subsequent surgeries the sutures holding the prosthetic device in place must be removed and a new device inserted and resutured to the surrounding tissue. After a number of replacements, the tissue surrounding the valve becomes perforated and scarred making attachment of each new valve progressively more difficult. See in this regard: